Pornpawee Chochuwong stunned Ratchanok Intanon to begin her HSBC BWF World Tour Finals campaign with a tough three-game victory at Impact Arena, Muang Thong Thani, on Wednesday night.
The world No.13 battled back to upset Ratchanok for her first-ever victory against her fifth-ranked opponent and claim her opening Group B win in the women's singles 15-21, 21-11, 21-18.
"I was very nervous in the first game. It's my first time playing in the World Tour Finals," said Pornpawee. "Fortunately I was able to regroup and play my own game and that's why I could come back.
"I think Ratchanok felt the pressure more in the third game and made some mistakes."
Pornpawee will have another tough assignment on Thursday against top-ranked Tai Tzu-ying in her second round-robin match of the US$1.5 million (approximately 45,150,000 baht) season finale while Ratchanok will play Pusarla V Sindhu of India.
Tai also needed three games to defeat seventh-ranked Sindhu in the other Group B match on Wednesday.
Pornpawee Chochuwong plays a shot against Ratchanok Intanon during their Group B match.
Tai, 26, lost the first game against Sindhu, the Rio Olympics silver-medallist, before regrouping to win it 19-21, 21-12, 21-17 in 59 minutes.
"It was a good match, there were no easy points," said Tai, who defeated Ratchanok in the Toyota Thailand Open semi-finals on Saturday.
In the mixed doubles category, Dechapol Puavaranukroh and Sapsiree Taerattanachai defeated Praveen Jordan and Melati Daeva Oktavianti of Indonesia 21-13, 16-21, 21-11 in an opening Group A clash.
They will play Seo Seung-Jae and Chae Yu-Jung of South Korea in the second match on Thursday.
The Thai world No.3 pair are chasing a hat-trick of titles in the Asian leg in Bangkok after they won the Yonex Thailand Open and the Toyota Thailand Open -- both World Tour Super 1000 event -- in consecutive weeks, heading into the finale.
World No.11 pair Jongkolphan Kititharakul and Rawinda Prajongjai got off to a winning start after they defeated the 18th-ranked Chloe Birch and Lauren Smith of England 21-10, 21-11 in the women's doubles Group B clash.
They will next play Kim So-Yeong and Kong Hee-Yong of South Korea, who also won their opener, in the second group match today.
Kim and Kong defeated Linda Efler and Isabel Herttrich of Germany in the other match-up 21-11, 19-21, 21-11.
"We didn't feel any pressure today although we played them for the first time today," said Jongkolphan.
"It was good that we won our opening match. A great boost for our confidence."
In Group A, Greysia Polii and Apriyani Rahayu beat Lee So-Hee and Shin Seung-Chan of South Korea 21-17, 22-24, 21-15 while Chow Mei Kuan and Lee Meng Yean won an all-Malaysian tie against Vivian Hoo and Yap Cheng Wen 21-16, 10-21, 21-15.
Polii and Rahayu will face Hoo and Wen while Lee and Shin play Chow and Lee today.
Elsewhere Spain's Carolina Marin, looking for her third straight trophy in as many weeks, dispatched Russia's Evgeniya Kosetskaya 21-19, 21-14 in a Group A action.
"The first game was tough after winning two back-to-back titles," said the reigning Olympic champion.
"Sometimes for your mindset, it's tough to be ready again for a new week, a new match and a new tournament."
Korean rising star An Se-Young also won her Group A opener, beating Michelle Li of Canada 21-12, 21-17.
Marin, who won both the Yonex Thailand Open and the Toyota Thailand Open, will play her second match against Li today while An will meet Kosetskaya.
In the men's singles action, Wang Tzu-wei of Taiwan defeated Angus Ng Ka Long of Hong Kong 21-10, 21-14 in Group B opener while Chou Tien-chen, also from Taiwan, came through his Group A opener against Lee Zii Jia of Malaysia 21-16, 12-21, 21-11.
The top eight players in each discipline are divided in two groups, playing in a round-robin format. The top two players advance to the semi-finals.